Gas Chromatography (GC) is used to resolve a mixture into its various components according to retention profiles of the different molecules passing through the GC column. While the technique can separate mixtures containing hundreds of substances, identifying the molecules that elute from the column is more problematic. To address the need for rapid and sensitive identification of the molecular species present, GC has been integrated with techniques such as mass spectrometry (MS) or Fourier transform infrared (FTIR) spectrometry.
Gas chromatography-mass spectrometry (GC-MS) is probably the most widespread tandem technique in the analytical instrumentation industry today. GC-MS systems are versatile and are employed across many different industries, particularly for environmental, chemical, petroleum, pharmaceutical, and toxicological applications. While GC-MS is a fast, sensitive technique suitable for multiple component detection and spectral identification, capable of measuring atomic species and supported by large available spectral libraries, it suffers from many disadvantages. These include compound separation to prevent MS interferences, non-linear calibrations, poor precision and accuracy (requiring constant calibration) and limited dynamic range. Problems also are encountered when high concentrations are present that can allow for chemical ionization to occur, generating questionable data.
To prevent MS spectral overlaps and interferences, the technique typically requires fully or nearly fully resolved GC peaks, with limited to no co-elution. Also GC-MS cannot differentiate between structural isomers that have identical electron impact and chemical ionization mass spectra. Moreover, most GC-MS systems require user selection of a list of compounds prior to analysis (e.g., approximately 60) and then only report those. Although the MS software can then do a global search and try to identify other peaks, it can seldom perform a quantitative analysis. This may be due, at least in part, to the fact that, although extensive (10,000 s), MS libraries are only qualitative and can differ from one MS manufacturer to another. Thus unless the MS is calibrated for a compound, a semi-quantitative analysis remains the best outcome for a detected peak.
GC-MS systems are also somewhat temperamental. For analysis, GC-MS normally requires helium or hydrogen gases, which raise cost and/or safety considerations. Equipment problems can arise with atmospheric leaks due to low operating pressures and, in general, GC-MS systems tend to require frequent maintenance, leading to extensive downtime. Then, bringing the systems back on-line can be time consuming and labor intensive.
While GC-MS is the more commonly deployed solution, Gas Chromatography-Fourier Transform Infrared Spectrometry (GC-FTIR) provides a powerful analytical tool that is particularly useful to distinguish among structural isomers that have identical electron impact and chemical ionization mass spectra.